An electro-acoustic microphone having a single planar diaphragm in a free field exhibits a null-response in the plane of its diaphragm, while having its maximum response along its perpendicular axis of symmetry. Such an ideal device is called a "cosine microphone". At any point between the axis and the plane of the diaphragm, the response is proportional to the cosine of the angle, say .theta., that the point is at w.r.t. the axis-centre. The reason for a response null in the plane of the diaphragm, of course, is that sound pressure impinges equally on either side thereof in opposite directions, thereby not causing movement of the diaphragm, given perfect symmetry in a free field. Practical microphones have relatively small diaphragms. Even sound pressure emanating from a point of maximum response far along the axis is bound to also reach the other side of the diaphragm with some phase shift and thereby cause some cancellation in the response.
It is desirable in many applications to have good cancellation in the plane of the diaphragm, yet to maintain high sensitivity on either side thereof, particularly in the vicinity of the axis. Such desirable characteristics would result in a conference apparatus having a reduced degree of voice switching, thereby enabling more natural two-way communication betweeen two or more parties of conferees. It is also desirable to have a frequency response that is relatively independant of source position (this means quality of response is independent of the talker's position).
U.S. Pat. No. 4,237,339 issued Dec. 2, 1980 to Bunting et al and assigned to The Post Office, London, England is an example of the use to which such bidirectional microphones are put for purposes of audio teleconferencing. The patent discloses an electro-acoustic terminal unit for use in an audio teleconferencing system comprising a loudspeaker and one or more microphones each having a sensitivity which is directionally dependent and exhibits at least one null or substantially null position. The loudspeaker and microphones are rigidly mounted on a boom and the microphones are so located and orientated relative to the loudspeaker that the null position is directed towards the loudspeaker.
In FIG. 3 of the above patent to Bunting et al, two "shallow" voice switches (35 and 44) are used in order to eliminate undesirable feedback between the loudspeaker 10 and the microphones 12 and 14. The permitted degree of "shallowness" of the voice switching is clearly dependent on the degree of isolation provided by the microphone.
It is possible to utilize complicated microphones to provide acceptable isolation. For instance, U.S. Pat. No. 3,573,399 issued Apr. 6, 1971 to Schroeder et al and assigned to Bell Telephone Laboratories, Incorporated, N.J., U.S.A. discloses the structure of a microphone having toroidal characteristics. The microphone is constructed from a plurality of concentric transducer elements, the outputs of which are combined in accordance with a predetermined formula.
It is, therefore, generally recognized in the field of teleconferencing as desirable to have microphones of sufficient sensitivity to pick-up distance talkers and conferees, while simultaneously providing good directionality to lessen pick-up of background noise and reverberation and to prevent feedback with a minimum of voice switching.